Understanding the Influence of Bio-Based Fertilizers (BBFs) on Sorption of Pharmaceuticals in Soils: Effects of pH and Organic Matter

Understanding the impact of bio-based fertilizers (BBFs) on the sorption of ionizable pharmaceuticals in soils via both pH and organic matter has received little research attention so far. To address this knowledge gap, we conducted batch sorption experiments with four common ionic pharmaceuticals used in human medicine (naproxen, diclofenac, furosemide, and ibuprofen), four types of commercial BBFs (ash-based, plant-based, animal by-product-based, and manure-based), and two agricultural soils from contrasting climates (Finland and Spain). The sorption affinity of the four pharmaceuticals varied across soil matrices. The addition of BBFs to soil caused modifications in both organic matter content (OMC) and pH levels, which reduced sorption of the pharmaceuticals in soils amended with the ash-based BBF, whereas BBFs rich in organic matter increased sorption. To further quantify the individual contributions of pH and OMC to sorption variation, a normalized sensitivity index (NSI) approach was employed using a sorption model parameterized by pH and OMC. NSI analysis revealed that the sorption of the studied compounds to soil was primarily pH-sensitive at low pH and in low-OMC zones. Dominance maps constructed from NSI values highlighted soil-specific transition zones in which pH or OMC governed sorption behavior in soil. These findings underscore the importance of compound-specific, soil-dependent BBF effects, and suggest that tailored management of soil properties through BBF application may offer effective strategies for mitigating pharmaceutical mobility in agroecosystems.